An important trend in consumer washing practices in recent years has been the move towards machine washing of clothes and other household articles at lower wash temperatures, i.e. less than about 30.degree. C. This has been occasioned both by the increased usage of coloured synthetic articles which may be adversely affected by prolonged washing at high temperatures, and also by energy conservation and other economic conditions. In turn, the trend towards low temperature washing has necessitated improvement in the detergency characteristics of washing machine compositions, particularly in the area of stain removal of bleachable-type stains such as tea, coffee, wine etc. Thus, it is well-known that improvements in bleachable stain removal performance at low wash temperatures are particularly difficult to effect because conventional peroxy bleach additives have only a limited solubility in water in this temperature region and ideally require boil conditions for optimum performance.
It has now been discovered, however, that the overall detergency performance and especially the stain removal performance, of conventional, granular, heavy-duty detergent compositions can be significantly improved at temperatures of 30.degree. C. and below by the use of a specified low level of water soluble magnesium salt, and particularly by the combined use of the water-soluble magnesium salt with certain aminopolyphosphonic acid materials, preferably when used together with a specific magnesium insensitive surfactant system. It is, of course, well-known to add magnesium compounds, and especially magnesium silicate, to bleach-containing detergent compositions at levels of about 2% or more in order to improve the stability of the bleach or in order to minimise interactions between bleach and other components such as enzymes. British Pat. No. 1,252,298 is representative of this approach. Such detergent compositions are characterized by having improved bleachable stain removal performance under boil-wash conditions where degradation of the bleach by, for instance, transition metal impurities is often a significant problem. U.S. Pat. No. 3,714,050, on the other hand, describes the use of high levels of magnesium sulfate for improving the storage stability of formulations containing bleach and enzyme.
There has apparently been no realization in the art, however, of the beneficial detergency and stain removal effects which can be obtained at very low wash temperatures, where peroxy bleaches are substantially inoperative, by the use of low levels of water-soluble magnesium salts in combination with low levels of aminopolyphosphonic acid materials, preferably together with magnesium-insensitive surfactant systems. The selection of the magnesium level is apparently of key importance because the addition of a soluble magnesium salt effectively adds hardness ions to the wash solution and, if added in conventional amounts, this can lead to a loss rather than an improvement in detergency performance, particularly in the areas of clay-soil removal and whiteness maintenance.
The present invention thus provides a detergent composition having improved detergency performance at low wash temperatures, particularly on bleachable-type stains and which delivers excellent cleaning results across the range of hardness and usage conditions without detriment to performance in the areas of particulate soil removal and whiteness maintenance.